Seattle and Portland could see triple-digit temperatures, something extremely rare for the Pacific Northwest.

Summers in the Pacific Northwest, historically, are extremely mild. In Seattle, the region’s largest urban area, average summer temperatures generally hover around 70°F. In fact, in 130 years of recording-keeping, the city has only reached triple-digits three times: in 1941, 1994, and 2009.

This week, however, it’s looking like Seattle could add another record to the books, with temperatures projected to get dangerously close to the 100°F mark on Wednesday and Thursday. To the south, Oregon is looking at temperatures as high as 107°F in Portland, the second-largest city in the Pacific Northwest.

The Pacific Northwest is the most recent region to face historic temperatures this summer, following a massive heatwave in the Southwest that grounded some airplanes in late June.

Unlike the Southwest, however, which regularly sees temperatures soar above 90°F, the relatively mild summers of the Pacific Northwest mean many residents are ill-prepared for a days-long stretch of high temperatures. Only 15 percent of homes in Seattle have central air conditioning, meaning that many residents will find it difficult, if not impossible, to escape the heat.

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“This is definitely not a town that was built on air conditioning, and usually we don’t need it,” Dana Felton, a meteorologist with the National Weather Service in Seattle, told the Seattle Times. “On average we have only three 90-degree-plus days a year.”

According to the New York Times, cities like Seattle and Portland are preparing for the heat by opening cooling centers for residents that don’t have access to air conditioning. Still, in Multnomah County — which includes Portland’s 639,863 residents — only three cooling centers are planned.

To make the situation even more dangerous for those prone to heat-related illness — the very old, the very young, or those with pre-existing medical conditions — nighttime temperatures in the region aren’t expected to drop too far below 70°F. When nighttime temperatures remain high, it’s even more difficult for the body to cool off, increasing the risk of heat-related illness.

Globally, climate change has been increasingly the likelihood of both record-high daytime temperatures and high nighttime temperatures. According to a 2011 analysis by Climate Central, nighttime temperatures have actually increased more rapidly than daytime temperatures since 2000. And the likelihood of an extreme heatwave occurring has increased due to climate change. According to a study published in Nature Climate Change earlier this summer, nearly a quarter of the world’s population will be exposed to deadly heat by the end of the century for at least 20 days a year, if drastic actions are not taken to cut carbon emissions.

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Even under a best-case scenario, making deep cuts to carbon emissions, nearly half of the world’s population will still experience 20 days of deadly heat by the end of the century.

Dangerous heat is likely to be even worse in urban areas, which suffer from something called the “heat island effect” — a phenomenon where urban temperatures are, on average, almost 2–5.5°F warmer than surrounding areas. As ThinkProgress’ Sam Fullwood noted in a July, the fact that urban areas are likely to be hardest hit by climate-fueled heat waves means that the nation’s most vulnerable communities — the poor, homeless, elderly, and communities of color, that are most densely concentrated in U.S. cities — are likely to face some of the most deadly impacts of climate change.